Re: simple-audio-card vs. platform question

From: Ricard Wanderlof <ricard.wanderlof@axis.com>
To: Lars-Peter Clausen <lars@metafoo.de>
Cc: "alsa-devel@alsa-project.org" <alsa-devel@alsa-project.org>
Subject: Re: simple-audio-card vs. platform question
Date: Thu, 29 Jan 2015 09:58:11 +0100	[thread overview]
Message-ID: <alpine.DEB.2.02.1501290938240.11532@lnxricardw1.se.axis.com> (raw)
In-Reply-To: <54C7D4D8.9020301@metafoo.de>

On Tue, 27 Jan 2015, Lars-Peter Clausen wrote:

> >> Usually in such a setup the CPU DAI driver also register the platform
> >> component. Which then as the same of_node as the CPU DAI and so the matching
> >> works. There are plenty of examples e.g. check the drivers using
> >> devm_snd_dmaengine_pcm_register().
> >
> > Curious though, wouldn't it be more natural to bind it all together in the
> > simple-audio-card DT entry, having a "simple-audio-card,platform"
> > specification? Or is the idea that normally the CPU DAI driver is fairly
> > tightly coupled to the PCM driver so it makes more sense to make that
> > connection in the code?
> 
> Usually there is no representation of the platform object in the devicetree. 
> E.g. typically this is a external shared DMA controller which is referenced 
> by the dmas property in the CPU DAI node.

But perhaps there should be? As it is now, the I2S driver is bound in the
code to the PCM driver using the devm_snd_dmaengine_pcm_register() call,
which doesn't seem to fit in with the device tree philosphy of having the 
DT describe the hardware.

Taking davinci-mcasp.c as an example, there's a bulk of #if's which govern
calling the correct PCM driver depending on the setup, which is a bit 
cumbersome.

Granted, having a platform description in the DT would not actually 
describe any specific hardware since as you said the DMA controller is 
normally described elsewhere, which would go against the DT philosophy.

I'm not necessarily saying it would be better with a platform description 
in the DT, all things considered, just trying to understand why it may not 
be.

BTW, I can't find any DT in the kernel where the DMA is referenced in the 
CPU DAI node, all the DT's I can find which use simple-audio-card at least 
just reference the i2s device, which in turn references the DMA. Makes 
sense to me, perhaps that is what you meant?

/Ricard
-- 
Ricard Wolf Wanderlöf                           ricardw(at)axis.com
Axis Communications AB, Lund, Sweden            www.axis.com
Phone +46 46 272 2016                           Fax +46 46 13 61 30